I am working on word endings in Latin, and I came across the word Ago. And I was looking at the different conjugations for this word and it did not make sense to me.


Endings are:

         o/m   |   mus

         s     |   tis

         t     |   nt

But, when ago is conjugated, this happens:

        Ago | Agimus 

       Agis | Agitis          

       Agit | Agunt

Why is it conjugated with an i?

  • 7
    It belongs to the third conjugation. Most verbs in the 3rd conjugation form the different persons of the present tense the same: -o, -is, -it, -imus, -itis, -unt. Thus lego-legis, scribo-scribis, etc. Or do you mean how did the 3rd conjugation evolve to get to that?
    – Rafael
    Oct 5, 2016 at 19:10
  • 2
    @Rafael, I think the question is why the third declension uses -i- (and -u-!) or how things evolved that way. A simple answer is "to avoid consonants in the endings colliding with consonants in the stem", but I doubt that would be satisfying.
    – Joonas Ilmavirta
    Oct 5, 2016 at 21:02
  • 1
    To keep it simple, 3rd conjugation is ALMOST* always: {an O, 4 I's, and a U}. Hope this helps
    – Luke H
    Feb 12 at 2:52

2 Answers 2


If you're asking how the third conjugation historically came to use the vowels i/u, it has to do with regular sound changes that affected the original Proto-Indo-European (PIE) endings.

The present indicative active paradigm for this verb in PIE, with its descendant paradigm in Latin, would look something like this:

1sg. *h₂eǵ-ō > agō
2sg. *h₂eǵ-e-si > agis
3sg. *h₂eǵ-e-ti > agit
1pl. *h₂eǵ-o-mos > agimus
2pl. *h₂eǵ-e-tes > agitis
3pl. *h₂eǵ-o-nti > agunt

In PIE, as you can see, the personal endings were preceded by a so-called "thematic vowel", either e or o (except in the 1sg.). This vowel then underwent the following changes in Latin:

  1. In non-initial open syllables, all vowels (basically; there are some exceptions) in Latin became i. This is the source of the i in agimus, agitis.
  2. e became i before a word-final consonant. This is what happened in agis, agit, where first the final -i of the PIE form was lost (as also in the 3pl.), leaving the s/t word-final.
  3. o became u before a nasal (again not absolutely always, but in many cases). This gives the u of agunt.
  • While I doubt that this what the OP was wondering about, this is too great not to upvote :)
    – brianpck
    Oct 6, 2016 at 13:26
  • 1
    @TKR, Although it doesn't help me conjugate the verb, I can now understand why in the world it would suddenly conjugate to something like 'agit'. (I know I will learn to conjugate more effectively later on, so that is not really a big deal!) Thanks so much for the input - I truly appreciate it. Oct 7, 2016 at 13:11

To ask “why” is inevitably a question about etymology and Indo-European (IE) comparative linguistics. The Latin 3rd conjugation continues IE thematic presents. There is a root, then the “thematic vowel” *e or *o, and then the personal ending. In Latin *e regularly becomes /i/ before /s/ and /t/. Thus IE *bher-e-ti becomes bharati in Sanskrit, but *ferit > fert in Latin, while *bher-o-nti becomes Sanskrit bharanti, and Latin ferunt. Does that make it any clearer? I am not sure that it does. Reconstructed languages are just as inscrutable as real ones.


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